Difluoromethane (known in the art by the abbreviation F32 or HFC-32) is one of the possible replacements for chlorofluorocarbons (CFC) with which the Montreal Protocol is concerned. It is more particularly intended to replace chloropenta-fluoroethane (F115, whose action on ozone is accompanied by a very strong contribution towards the greenhouse effect) and, in the near future, F22 or chlorodifluoromethane. In this respect, it forms part of the composition of several mixtures of quasi-azeotropic nature, such as R407 C (mixture with HFC-125 or pentafluoroethane in a proportion of 50%/50 by weight) or R410 A (HFC-32/HFC-125 or pentafluoroethane/HFC-134a or 1,1,1,2-tetrafluoroethane mixture in a proportion of 23%/25%/52% by weight), which are used in the refrigeration industry.
F32 can be obtained by fluorination of methylene chloride (CH2Cl2) using hydrogen fluoride (HF) in the presence of a catalyst, or by hydrogenolysis of dichlorodifluoromethane (F12) or chlorodifluoromethane (F22), or alternatively by decomposition, in the presence of HF, of α-fluoro ethers under the action of Lewis acids.
Some of these processes require acidic or basic washes which introduce larger or smaller amounts of water into the final product. This product must thus undergo an additional drying operation in order to satisfy the specifications normally set for hydrofluorocarbons (HFCs), i.e. less than 10 ppm water. Such a specification is required in order to avoid problems of corrosion in refrigeration machines.
Molecular sieves, also known as synthetic zeolites, are chemical compounds widely used in the industry as adsorbing agents, in particular for drying gases or liquids. There are metallic aluminosilicates with a three-dimensional crystal structure consisting of an assembly of tetrahedra. These tetrahedra are formed by four oxygen atoms which occupy the peaks, and which surround either a silicon atom or an aluminium atom placed at the centre. These structures generally contain cations to make the system electrically neutral, such as those derived from sodium, potassium or calcium.
In the case of molecular sieves, of the so-called A type, the tetrahedra are assembled such that they compose a truncated octahedron. These octahedra are themselves arranged in a simple cubic crystal structure, forming a network with cavities approximately 11.5 Å in diameter. These cavities are accessible via apertures, or pores, which can be partially blocked by means of cations. When these cations are derived from sodium, these cavities have an aperture diameter of 4.1 Å, and this thus gives a so-called 4 A molecular sieve. The crystal structure of such a sieve can be represented by the following chemical formula:Na12[(AlO2)12(SiO2)12].X H2O
in which X, which represents the number of molecules of water forming part of the structure (water of crystallization), can be up to 27, which represents 28.5% by weight of the anhydrous zeolite.
After removal of the water of crystallization by heating to a temperature of about 500 to 700° C., the cavities in these substances are available for the selective adsorption of various gases or liquids. Thus, the pores in the various types of zeolite allow passage and adsorption, in the corresponding cavities, only of molecules whose effective diameter is less than or equal to the effective diameter of the pores. In the case of the drying of gases or liquids, it is thus water molecules which are retained by selective adsorption inside the abovementioned cavities, while the substance to be dried is itself not or only negligibly adsorbed.
The size of the apertures (or pores) can, moreover, be modified according to the different types of molecular sieve. Thus, by exchanging most of the sodium ions of a 4A molecular sieve for potassium ions, the 3A molecular sieve is obtained, the pores of which have a diameter of about 3 Å. The 5A molecular sieve is prepared by replacing the sodium ions with calcium ions, the effective diameter of the pores then being about 5 Å.
Sieves of 3A, 4A or 5A type are widely commercially available.
In practical terms, the molecular sieves can be combined with other substances such as binders, in particular clays, and the compositions obtained are shaped, for example, into granules, beads or extrudates.
The molecular sieves thus conditioned are used industrially by loading into drying columns, into which the wet gas is introduced, and from which it emerges dried.
After a certain period of running in a drying column, which varies with the operating conditions (flow rate of gas to be dried, amount of molecular sieve), an increase in the water content of the dried gas leaving the column is observed. This moment corresponds to the obtainment of the water-saturation capacity of the sieve feed stock, i.e. the maximum amount of water which can be adsorbed. This amount is generally about 20% by weight, expressed relative to the weight of dry sieve.
The sieve feed stock thus saturated with water must then be subjected to a so-called regeneration treatment, after which the initial capacity of the sieve to adsorb water is restored. This treatment usually consists in passing a stream of an inert gas, at a temperature of between 200° C. and 300° C., into the column. In practical terms, this treatment of the saturated sieve feed stock is carried out in the same column as that in which the stream of gas to be dried was introduced. The same drying column thus functions occasionally in a phase of drying the wet gas, and occasionally in a phase of regenerating the molecular sieve feed stock with the inert gas. However, after a certain number of these drying-regeneration cycles, an irreversible decrease in the water-saturation capacity of the sieve feed stock is observed, and it is then necessary to stop running the column so as to renew the sieve feed stock with a fresh feed stock.
In the present text, the expression “fresh sieve feed stock” means a sieve feed stock which has not been used as a drying agent.
Under the conditions of the industrial practice of drying gases using molecular sieves, 2 drying columns are usually used, which can run alternately, one being in the drying phase while the other is in the regenerating phase.
The drying of F32 with molecular sieves poses a specific problem on account of the proximity of effective diameter between the molecules of F32 and of water (0.33 nm and 0.21 nm respectively).
Thus, patent application FR 2,705,586 clearly mentions the placing in contact, in a pressurized container, of wet F32 with a 3A type molecular sieve and an ester oil at a temperature of 120° C.
However, that document teaches that, under these conditions, the F32 is adsorbed onto the said sieve and undergoes a decomposition reaction, the effect of which is, via a modification of the sieve's crystal state, to greatly reduce its water-saturation capacity.
That document concludes that such a sieve is not suitable for use as an agent for drying F32. The patent application consequently recommends, with the aim of drying F32 circulating as a refrigerant inside a refrigeration machine, a molecular sieve obtained by a complementary treatment of a 3A type sieve which results in a decrease in the size of the pores.